Build SourceKit on Linux if dispatch is present

SourceKit can be built on Linux if dispatch is available
so enable this so it happens by default.

Issue: SR-1676

CMakeLists.txt

@@ -363,8 +363,14 @@ if("${CMAKE_SYSTEM_NAME}" STREQUAL "")
 endif()
 if("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
   set(SWIFT_BUILD_SOURCEKIT_default TRUE)
+elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux")
+  if(EXISTS ${SWIFT_PATH_TO_LIBDISPATCH_SOURCE})
+    set(SWIFT_BUILD_SOURCEKIT_default TRUE)
+  else()
+    set(SWIFT_BUILD_SOURCEKIT_default FALSE)
+  endif()
 else()
-  set(SWIFT_BUILD_SOURCEKIT_default FALSE)
+    set(SWIFT_BUILD_SOURCEKIT_default FALSE)
 endif()
 option(SWIFT_BUILD_SOURCEKIT
     "Build SourceKit"

tools/SourceKit/CMakeLists.txt

@@ -509,7 +509,7 @@ if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux")
     add_custom_command(
       OUTPUT "${SWIFT_PATH_TO_LIBDISPATCH_BUILD}/src/.libs/libdispatch.so"
       COMMAND autoreconf -fvi
-      COMMAND /usr/bin/env "CC=${PATH_TO_CLANG_BUILD}/bin/clang" "CXX=${PATH_TO_CLANG_BUILD}/bin/clang++" ${SWIFT_PATH_TO_LIBDISPATCH_SOURCE}/configure --prefix="${CMAKE_INSTALL_PREFIX}"
+      COMMAND /usr/bin/env "CC=${PATH_TO_CLANG_BUILD}/bin/clang" "CXX=${PATH_TO_CLANG_BUILD}/bin/clang++" "OBJC=${PATH_TO_CLANG_BUILD}/bin/clang" ${SWIFT_PATH_TO_LIBDISPATCH_SOURCE}/configure --prefix="${CMAKE_INSTALL_PREFIX}"
       COMMAND /usr/bin/make
       COMMAND /bin/mkdir -p ${SWIFT_PATH_TO_LIBDISPATCH_BUILD}/src/.libs/
       COMMAND /bin/cp ${SWIFT_PATH_TO_LIBDISPATCH_SOURCE}/src/.libs/libdispatch.so ${SWIFT_PATH_TO_LIBDISPATCH_BUILD}/src/.libs/libdispatch.so